Law 4 (The Players' Equipment) states very clearly that "A player must not use equipment or wear anything which is dangerous to himself or another player (including any kind of jewelry)." In addition, FIFA included in the 2002/2003 edition of the Laws of the Game a section on "Additional Instructions for Referees, Assistant Referees and Fourth Officials" in which this requirement is further emphasized: "Referees are reminded that, in accordance with Law 4, players may not wear any kind of jewelry."
REFEREES OFFICIATING IN PROFESSIONAL MATCHES MUST ENSURE THAT THIS CLEAR RESTRICTION IS PROPERLY ENFORCED!
USSF reaffirms its advice to referees that jewelry worn solely for medical purposes may be permitted but only if, in the opinion of the referee, the item is not dangerous. Such items can often be worn safely if appropriately taped. Additionally, for married players, a wedding ring may also be permitted if it does not include any dangerous projections. An item of jewelry permitted by the referee under these provisions must have been carefully inspected prior to the commencement of the match.
The match must not be permitted to start with any player wearing illegal equipment or apparel, including jewelry. Players who insist on retaining such items will not be permitted to participate in the match. Any player who, having been instructed to correct illegal equipment, nevertheless attempts to participate in play without having made the correction is subject to being cautioned for dissent.
The referee, assistant referees, and the fourth official all share in the responsibility to enforce the clear requirements of Law 4 related to jewelry and are advised to discuss in their pregame meeting specific measures each will take to ensure compliance prior to as well as throughout the match.